1. The anostracan fairy shrimp Branchinecta gaini inhabits one of the most hostile environments on earth, living in pools and lakes in Antarctica. Between January 2002 and January 2003 temperatures in two pools where B. gaini are extremely abundant on Adelaide Island ranged from −18.6 to −15.7 °C in winter, to 19.4 to 17.1 °C in summer, whilst air temperatures ranged from −34 to 6.3 °C.\ud \ud 2. Branchinecta gaini survives winter as cysts, but endures large summer temperature fluctuations as adults. Cysts froze between −24.4 and −25.7 °C. In experiments adults survived 0-10 °C with no mortality for 1 week, 25 °C for nearly 48 h with 50% mortality, and at 32 °C complete mortality occurred in <1 h.\ud \ud 3. Oxygen consumption (M˙O2) in B. gaini approximately doubled for every 10 °C temperature rise (Q10 = 2.04) up to 20 °C where it reached a peak. Females had, on average 19% higher M˙O2 than males. Females also had greater metabolic scopes, (maximum-minimum M˙O2 across temperatures was ×3.6 for females, ×3.1 for males).\ud \ud 4. Ventilation frequency increased linearly with temperature, and did not decline at 25 °C, indicating animals were `trying' progressively harder to supply oxygen to tissues, and oxygen deficiency was the probable cause of death. Females had a higher ventilation frequency than males (8.6-17.1% higher) and they also exhibited greater scope to raise ventilation frequency (×2.4 for females versus ×1.5 for males).\ud \ud 5. Great metabolic flexibility allows B. gaini to exploit extreme, highly fluctuating environments, and larger ventilatory and respiratory scopes allow females to survive higher temperatures than males. Because of this flexibility their prospects for coping with physical environmental change are high
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